SNIFTIRS

Li PC, EC, DME /LiAsF, SNIFTIR, EMIRS R0C02Li, Li2C03, peroxides, ketals [181]... 

QCMB RAM SBR SEI SEM SERS SFL SHE SLI SNIFTIRS quartz crystal microbalance rechargeable alkaline manganese dioxide-zinc styrene-butadiene rubber solid electrolyte interphase scanning electron microscopy surface enhanced Raman spectroscopy sulfolane-based electrolyte standard hydrogen electrode starter-light-ignition subtractively normalized interfacial Fourier transform infrared... 

According to experimental data,208,209 the SNIFTIR technique can be used to probe the electrical properties of the electrical double layer even in more concentrated solutions where cyclic voltammetry (cv), impedance, chronocoulometry, and other techniques are not applicable. Iwasita and Xia210 have used FTIR reflection-adsorption spectra to identify the potential at which the orientation of water molecules changes from hydrogen down to oxygen down. 

Both methods obtain the necessary sensitivity by modulating the electrode potential between two values which define two distinct states of the electrode surface thus the chemistry to be observed is directly modulated and may be detected with great sensitivity by an appropriate form of synchronous detection. In the case of EMIRS, the modulation frequency is made sufficiently high compared to the wavelength scanning rate to enable a phase sensitive detection system to be used whereas, for SNIFTIRS, the electrode potential is held for a sufficient period at each potential to accumulate data from several interferometric scans and, after an adequate number, the two sets of data are ratioed. 

In EMIRS and SNIFTIRS measurements the "inactive" s-polarlsed radiation is prevented from reaching the detector and the relative intensities of the vibrational bands observed in the spectra from the remaining p-polarised radiation are used to deduce the orientation of adsorbed molecules. It should be pointed out, however, that vibrational coupling to adsorbate/adsorbent charge transfer (11) and also w electrochemically activated Stark effect (7,12,13) can lead to apparent violations of the surface selection rule which can invalidate simple deductions of orientation. 

The surface actlve/surface inactive difference between p-polarlsed/ s-polarised radiation has enabled an alternative modulation technique, polarisation modulation, to be developed (15,16). In electrochemical applications, it allows surface specificity to be achieved whilst working at fixed potential and without electrochemical modulation of the interface. It can be implemented either on EMIRS or on SNIFTIRS spectrometers and can be very valuable in dealing with electrochemically irreversible systems however, the achievable sensitivity falls well short of that obtained with electrochemical modulation. It should also be noted that its "surface specificity" is not truly surface but extends out into the electrolyte with decreasing specificity to about half a wavelength. 

The EMIRS and SNIFTIRS methods provide the IR vibrational spectra (really the difference spectra - see later) of all species whose population changes either on the electrode surface or in the electrical double layer or in the diffusion layer in response to changing the electrode potential. Spectra will also be obtained for adsorbed species whose population does not change but which undergo a change in orientation or for which the electrode potential alters the Intensity, the position or shape of IR absorption bands. Shifts in band maxima with potential at constant coverage (d nax 6 very common for adsorbed species and they provide valuable information on the nature of adsorbate/absorbent bonding and hence also additional data on adsorbate orientation. 

It will be clear that EMIRS and SNIFTIRS spectra are difference spectra and can be somewhat complex ( ). Typically they will contain positive absorption bands from species present in excess at potential El compared to potential E2 and negative absorption bands from species whose polulation changes oppositely with potential. In addition, bands which shift with potential will appear as a single bipolar band either with one lobe of each sign, figure 2, (or even more complex structures with three lobes). 

The adsorption of Intact molecules Is encountered In many areas of electrochemistry. A complete description of the adsorbed state In terms of the orientation of the molecule, the way In which It bonds to the surface, the perturbation of the molecular structure caused by this additional bonding and the Interaction between adjacent molecules Is the ultimate goal of spectroscopic techniques. As more systems are studied by the EMIRS and SNIFTIRS methods, ways are being found to assess more of this Information. 

Figure 11. SNIFTIRS spectra from a Pt electrode in IM HClOi, + 0.5 mM p-difluorobenzene. Modulation from +0.2V to +0.4V.

It is only since 1980 that in situ spectroscopic techniques have been developed to obtain identification of the adsorbed intermediates and hence of reliable reaction mechanisms. These new infrared spectroscopic in situ techniques, such as electrochemically modulated infrared reflectance spectroscopy (EMIRS), which uses a dispersive spectrometer, Fourier transform infrared reflectance spectroscopy, or a subtractively normalized interfacial Fourier transform infrared reflectance spectroscopy (SNIFTIRS), have provided definitive proof for the presence of strongly adsorbed species (mainly adsorbed carbon monoxide) acting as catalytic poisons. " " Even though this chapter is not devoted to the description of in situ infrared techniques, it is useful to briefly note the advantages and limitations of such spectroscopic methods. 

Figure 6. SNIFTIR spectra of the adsorbed intermediates involved in the oxidation of 0.1 M CHjOH in 0.5 M HCIO4 on a smooth Pt electrode (p-polarized light modulation potential AE = 0.3 V averaging of 128 interferograms). Electrode potential (mV/RHE) (1) 370, (2) 470, (3) 570, (4) 670, (5) 770.

SNIFTIRS subtractively normalized interfacial Eourier transform infrared spectroscopy... 

Bowmaker et al. (1998) have also used IR from a synchrotron source in the SNIFTIRS mode to smdy cyanate adsorption on silver. The spectra are shown in Fig. 27.38. The far-IR spectrum shows a broad band at about 360 cm at electrode... 

Figure 6.18 Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) spectra of a polished polyciystaUine Pt electrode, immersed in 0.1 M HCIO4, + 5 M CH3OH electrolyte. All spectra were normahzed to the base spectrum collected at 0 mV vs. RHE. (Reproduced from Iwasita and Vielstich [1988].)...

Nichols RJ, Bewick A. 1988. SNIFTIRS with a flow cell the identification of the reaction intermediates in methanol oxidation atplatinum anodes. Electrochim Acta 33 1691-1694. 

Figure 11.8 (a) SNIFTIR spectra of the species coming from methanol adsorption and oxidation at a Pt/C electrode 0.1 M HCIO4 + 0.1 M CH3OH 25 °C. (h) SNIFTIR spectra of the species coming from ethanol adsorption and oxidation on a Pt/C electrode 0.1 M HCIO4 + 0.1 M C2H5OH 25 °C. 

Iwasita T., Rasch B., Cattaneo E., Vielstich W. 1989. A SNIFTIRS study of ethanol oxidation on platinum. Electrochim Acta 34 1073-1079. 

Fig. 5.35 SNIFTIRS spectrum from a polished Pt electrode in 0.5 m LiC104 in propylene carbonate. Reference potential 2.00 V versus Li/Li+ electrode working potential 3.20 V versus Li/Li+ electrode. According to P. Novak et al.

The IR investigations described in this presentation are based on the SPAIRS variant of SNIFTIRS. 

Fig. 1.5. Simplified block diagram of the experimental setup for SNIFTIRS.

Different experimental approaches were applied in the past [6, 45] and in recent years [23, 46] to study the nature of the organic residue. But the results or their interpretation have been contradictory. Even at present, the application of modem analytical techniques and optimized electrochemical instruments have led to different results and all three particles given above, namely HCO, COH and CO, have been recently discussed as possible methanol intermediates [14,15,23,46,47]. We shall present here the results of recent investigations on the electrochemical oxidation of methanol by application of electrochemical thermal desorption mass spectroscopy (ECTDMS) on-line mass spectroscopy, and Fourier Transform IR-reflection-absorption spectroscopy (SNIFTIRS). 

Most results of in situ IR studies on Pt in acidic methanol solutions so far have been obtained using a relatively fast (8.5-13.6) Hz) modulation of electrode potential. As already pointed out by Bockris [27], collection of spectral data alternatively at two potentials is not appropriate for processes which are not reversible to follow the change of potential. In this study the SPAIRS version of SNIFTIRS was performed by stepping the potential from a reference potential in the anodic direction, allowing sufficient time at each potential to reach stationary conditions. 

Fig. 2.11. Potential step SNIFTIRS spectra from a polycrystalline crystalline Pt electrode, in 10 2 M CH3OH/O.I M HCIO4. 2400-1700 cm 1 spectral region showing the changes in the bands for linear and multi bonded CO on Pt.

Fig. 2.12. Potential step SNIFTIRS spectra from a polished polycrystalline Pt electrode, in 3 M CHjOH/O.l M HC104. Reference spectrum taken at 50 mV. Other details as in Fig. 2.10.

The SNIFTIRS results presented here confirm the presence of formic acid and methyl formate as by-products of methanol oxidation. Other by-products such as formaldehyde could not be detected under our experimental conditions. In fact, formaldehyde hydrolyses (99.99%) in aqueous solutions to a gemdiol H2C(OH)2, and the typical aldehyde bands are, therefore, not expected. 

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